There are metal alloys, for example, some aluminum, magnesium and titanium alloys, that display exceptional ductility when deformed under controlled conditions. These aluminum alloys are susceptible to extensive deformation under relatively low shaping forces. Such alloys are characterized as being superplastic. The tensile ductility of superplastic metal alloys typically ranges from 200% to 1000% elongation.
Superplastic alloy sheets are formed by a variety of processes into articles of manufacture that are frequently of complex shape. These superplastic forming (SPF) processes are usually relatively slow, controlled deformation processes that yield complicated products. But an advantage of SPF processes is that they often permit the manufacture of large single parts that cannot be made by other processes such as conventional sheet metal stamping. Sometimes a single SPF part can replace an assembly of several parts made from non-SPF materials and processes.
There is a good background description of practical superplastic metal alloys and SPF processes by C. H. Hamilton and A. K. Ghosh entitled "Superplastic Sheet Forming" in Metals Handbook, Ninth Edition, Vol. 14, pages 852-868. In this text several suitably fine grained, superplastic aluminum and titanium alloys are described. Also described are a number of SPF processes and practices for forming superplastic materials. One practice that is adaptable to forming relatively large sheets of relatively low cost superplastic aluminum alloys into automobile body panels or the like is stretch forming.
As described, stretch forming comprises gripping or clamping the flat sheet blank at its edges, heating the sheet to its SPF temperature and subjecting one side to the pressure of a suitable gas such as air or argon. The central unclasped portion of the heated sheet is stretched and plastically deformed into conformity with a shaping surface such as a die cavity surface. The term "blow forming" applies where the working gas is at a superatmospheric pressure (for example, up to 690 to 3400 kPa or 100 psi to 500 psi). Vacuum forming describes the stretch forming practice where air is evacuated from one side of the sheet and the applied pressure on the other side is limited to atmospheric pressure, about 15 psi. As stated, the sheet and tools are heated to a suitable SPF condition for the alloy. For SPF aluminum alloys, this temperature is typically in the range of 400.degree. C. to 550.degree. C. The rate of pressurization is controlled so the strain rates induced in the sheet being deformed are consistent with the required elongation for part forming. Suitable strain rates are usually 0.0001 to 0.01 s.sup.-1.
In stretch forming, a blank is tightly clamped at its edges between complementary surfaces of opposing die members. A schematic example is shown in FIG. 9, p. 857 of the Hamilton et al article, supra. At least one of the die members has a cavity with a forming surface opposite one face of the sheet. The other die opposite the other face of the sheet forms a pressure chamber with the sheet as one wall to contain the working gas for the forming step. The dies and the sheet are maintained at an appropriate forming temperature. Electric resistance heating elements are located in press platens or sometimes embedded in ceramic or metal pressure plates located between the die members and the platens. A suitable pressurized gas such as air is gradually introduced into the die chamber on one side of the sheet, and the hot, relatively ductile sheet is stretched at a suitable strain rate until it is permanently reshaped against the forming surface of the opposite die. During the deformation of the sheet, gas is vented from the forming die chamber.
In the SPF stretch forming process, the periphery of the sheet is held in a fixed position between "binder surfaces" of the forming dies or tools. The binder surfaces of the dies grip the sheet in a gas tight seal and the sheet does not flow over the binder surface as is typical in a conventional deep drawing operation. It is common to use a raised land seal bead to grip the periphery of the sheet. FIG. 10, page 857 of the Hamilton et al article, supra, shows a trapezoidal bead machined into the otherwise flat binder surface of one of the SPF forming tools. The binder surface of the opposing tool may be machined flat as shown in FIG. 10(a), or it may be machined to have a complementary trapezoidal recess as shown in FIG. 10(b). More commonly, male rectangular cross-section beads are employed on one tool surface while the opposing binder surface is flat. A typical bead has a raised rectangular or trapezoidal cross-section approximately 10-15 millimeters wide and 0.5-1 mm tall.
A problem encountered in superplastic forming is the sticking of the formed sheet to the tool in the vicinity of the seal bead during part extraction. Because the sheet components are very deformable at the forming temperature, sticking can distort the panel during panel extraction. The problem is particularly acute with aluminum sheet and severely slows the effective removal of an SPF-formed part from the binder portions of the tools. Sticking between the aluminum sheet and the die faces occurs primarily on the raised bead face but also on the opposing flat face. The sticking is due to reaction of the die surfaces with freshly exposed, unoxidized aluminum.
This unoxidized, reactive aluminum is exposed at the sheet surface as a result of plastic deformation of the aluminum sheet during the clamping process prior to sheet forming. As the die is closed, aluminum is extruded (locally) away from the volume clamped between the bead and the opposing tool side. As a result, the protective aluminum oxide film on the aluminum sheet surface is ruptured, and highly reactive aluminum is brought into intimate contact with the tool surface. The SPF forming tools are often made of, e.g., 1020 steel, ductile cast iron or aluminum. For most such tool materials, local reaction or microwelding occurs which can locally bond the aluminum sheet to the tool and cause sticking and tearing during subsequent part removal.
This part sticking problem may be tolerable when low volume production parts can be carefully pried from the tool, but the problem cannot be tolerated when high production rates are required. To adapt SPF to the production of automotive panels, e.g., practices must be developed that facilitate fast removal of an SPF-formed part from the forming tools.